1. Field of the Invention
The present invention relates to a drive power transmission device for use in a motor vehicle, and more particularly to a drive power transmission device employing a viscous shear coupling.
2. Description of the Relevant Art
Viscous shear couplings are employed as the central differentials, for example, of four-wheel drive motor vehicles.
For example, a viscous shear coupling disclosed in U.S. Pat. No. 4,022,084 comprises coaxial input and output shafts, an enclosure disposed between the input and output shafts, and first and second groups of mutually interleaved plates housed in the enclosure and coupled respectively to the input and output shafts. The enclosure is filled with highly viscous silicone oil.
Where the disclosed viscous shear coupling is employed in the central differential of a four-wheel drive motor vehicle, it can eliminate the problem of tight turn braking which would otherwise be caused by the difference between the angular speeds of front and rear wheels as upon steering the motor vehicle at low speed into a garage or a parking space. During travel at high speed, however, the viscous shear coupling suffers a lower efficiency of torque transmission than a direct shaft coupling since the drive force is transmitted through the silicone oil at all times.